Standardization works of international mobile telecommunication (IMT)-advanced which is a next generation (i.e., post 3rd generation) mobile communication system are carried out in the international telecommunication union radio communication sector (ITU-R). The IMT-advanced aims at support of an Internet protocol (IP)-based multimedia service with a data transfer rate of 1 Gbps in a stationary or slowly moving state or 500 Mbps in a fast moving state.
3rd generation partnership project (3GPP) is a system standard satisfying requirements of the IMT-advanced, and prepares LTE-advanced which is an improved version of long term evolution (LTE) based on orthogonal frequency division multiple access (OFDMA)/single carrier-frequency division multiple access (SC-FDMA) transmission. The LTE-advanced is one of promising candidates for the IMT-advanced. A technology related to a relay station is one of main technologies for the LTE-advanced. The relay station is a device for relaying a signal between a base station and a terminal, and is used for cell coverage extension and throughput enhancement of a wireless communication system.
Further, in recent wireless communication system fields, there is ongoing research on performance improvement of a terminal located in a cell edge in a wireless communication system having a multi-cell structure. In particular, research on coordinated multiple-point transmission and reception (CoMP) is actively carried out. The CoMP is a technique for transmitting a signal through mutual cooperation between cells in the communication system having the multi-cell structure. According to the CoMP, a terminal, especially a terminal located in a cell edge, can communicate with a plurality of base station and thus can improve not only a transmit power gain but also signal sensitivity.
Representative methods of coordinated communication using multiple terminals include amplify and forward (AAF), demodulate and forward (DemAF), and decode and forward (DAF) methods. A hybrid DAF method is a method which combines these methods.
The hybrid DAF method determines whether to apply the DAF method and the AAF method according to whether decoding is successful after decoding a received signal. When operating in this manner, there is no problem in case of the DAF operation. However, in case of the AAF operation, there is a problem in that the AAF must be performed by storing the received signal until decoding of the received signal is complete to be able to know whether the decoding is successful. Once the received signal is stored, since the stored signal is not an analog signal but a digital signal obtained by quantizing the received signal, quantization noise may be added in a quantization process, and an additional storage space is required in a base station to store the signal. Therefore, it is actually impossible to apply the conventional hybrid DAF method in a format in which the AAF and the DAF are combined. Thus, when implemented in practice, the hybrid DAF method is implemented by combining the DAF and a modified DemAF in which a signal has multiple levels. Since the DemAF and the DAF both have a problem of error propagation, when implemented with the conventional hybrid DAF method, the error propagation problem still exists even if the hybrid DAF method is used.
Meanwhile, in coordinated communication using multiple terminals, a channel between a terminal which takes a relay role (hereinafter, such a terminal is referred to as a relay station) and a terminal which takes a source role may have better quality than a channel between the terminal and a base station, but it is a channel in which data must be protected by using a channel code or the like and in which an error may occur when the relay station cannot decode received data. In addition, according to a characteristic of most relay communication schemes which show optimal performance when the relay station is located at a middle point between the terminal and the base station, a part of data of the terminal is encoded and transmitted to the relay station, and the relay station decodes the data and re-encodes the data according to a situation of the relay station, and then transmits the data to the base station.
In the conventional coordinated communication, it is assumed a situation in which a channel environment is not significantly different between the terminal and the relay station or the base station. In particular, it is assumed a situation in which a channel environment between the terminal and the relay station is an environment which must use a channel code and a decoding error occurs in this environment. However, in most cases of coordinated communication, coordinated communication is made in general with a nearest terminal if possible under the condition that a channel in use is a channel having a small correlation between channels so as to obtain a diversity effect. Therefore, it can be assumed that a channel environment between the terminal and the relay station which participate in coordinated communication in a coordinated communication system is a clean environment in which an error does almost not occur even if a channel code is not present. Accordingly, there is a need for a method that performs coordinated communication without having to perform unnecessary decoding in such a situation.